| 日時: | 2010 年 1月 12日(火) 13:30 - 15:00 (レクチャー1) |
| 場所: |
神戸大自然科学総合研究棟3号館609号室 |
| 講演者: | Sarah T. Stewart-Mukhopadhyay (ハーバード大) |
| タイトル: | Open questions about the formation of the largest impact craters |
| abstract: | Impact craters are the most common landform on planetary surfaces;
however, the mechanics of the end stages of the formation of large
craters is not fully understood. The final stage of crater formation
involves the collapse of a hemispherical transient cavity. Around small
craters, the limited amount of collapse preserves a bowl-shaped cavity.
In contrast, the observed shallow depths and complex inner morphologies
of large craters require very low shear strength in the collapsing
material. Because the observed amount of collapse cannot be reproduced
using quasi-static (laboratory) values for the frictional strength of
fractured rock, a temporary weakening mechanism is necessary. I will
present simulations that investigate the hypothesis that craters
collapse along a network of impact-generated faults that weaken during
long displacements at high slip velocities via, for example, frictional
melting. The model reproduces the major geologic features observed
around the largest terrestrial craters (Vredefort, Sudbury, and
Chicxulub), including shallow depths, fault structures, frictional melt
distributions, and deep-seated central uplifts. Finally, I will raise
open questions related to the formation of multi-ring impact basins. Reading: Senft, L. E., and S. T. Stewart. Dynamic Fault Weakening and the Formation of Large Impact Craters.
Earth and Planetary Science Letters, 287, 471-482, |
| 日時: | 2010 年 1月 12日(火) 15:00 - 16:30 (レクチャー2) |
| 場所: |
神戸大自然科学総合研究棟3号館609号室 |
| 講演者: | Sarah T. Stewart-Mukhopadhyay (ハーバード大) |
| タイトル: | Shock wave experiments in ice: a focus on phase changes |
| abstract: | Laboratory shock wave experiments provide fundamental data to understand
the response of ice during planetary impact cratering events. I will
describe laboratory measurements on phase changes in ice and discuss
their implications for impact-induced melting and vaporization on planets. Readings: a. Boslough, M.B. and J.R. Asay (1993) High-pressure Shock Compression of Solids, Springer-Verlag: New York, p. 7-42.
b. Stewart, S.T., and T. J. Ahrens. Shock Properties of H2O ice. Journal
of Geophysical Research - Planets, 110, E03055,
c. Stewart, S. T., A. Seifter, and A. W. Obst. Shocked H2O Ice: Thermal
Emission Measurements and the Criteria for Phase Changes during Impact
Events. Geophysical Research Letters, 35, L23203, |
| 日時: | 2010 年 1月 13日(水) 14:00 - 15:30 (レクチャー3) |
| 場所: |
神戸大自然科学総合研究棟3号館609号室 |
| 講演者: | Sarah T. Stewart-Mukhopadhyay (ハーバード大) |
| タイトル: | Impact cratering on icy satellites |
| abstract: | Impact craters on icy satellites display a wide range of morphologies,
some of which have no counterpart on rocky bodies. Numerical simulation
studies have struggled to reproduce the diversity of features, such as
central pits and transitions in crater depth with increasing diameter,
observed on the icy satellites around Jupiter. The transitions in crater
depth (at diameters of about 26 and 150 km on Ganymede and Callisto)
have been interpreted as reflecting subsurface structure. I will present
models of the formation of craters with diameters between 400 m and
about 200 km on Ganymede using different subsurface temperature
profiles. The calculations include recent improvements in the model
equation of state for H2O and quasi-static strength parameters for ice.
The shock-induced formation of dense high-pressure polymorphs (ices VI
and VII) creates a gap in the crater excavation flow, which I call
discontinuous excavation. For craters larger than about 20 km,
discontinuous excavation concentrates a hot plug of material (>270 K and
mostly on the melting curve) in the center of the crater floor. The size
and occurrence of the hot plug are in good agreement with the observed
characteristics of central pit craters, suggesting that a genetic link
exists between them. I will present depth vs. diameter curves for
different internal temperature profiles and discuss the implications for
subsurface structure. Reading: Modeling the morphological diversity of impact craters on icy satellites by L. E. Senft and S. T. Stewart, Icarus, in revision, 2009. |